1. Field of the Invention
The present invention relates generally to remedial well bore sealing methods, and more particularly, to improved methods of sealing openings such as channels or fractures communicating the well bore with a subterranean zone into which fluids are lost.
2. Description of the Prior Art
In the drilling of oil and gas wells using the rotary drilling method, drilling fluid is circulated through the drill string and drill bit and back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean zones through which the well bore is drilled and circulates cuttings out of the well bore. The drilling fluid includes gelled and particulate material which builds up on the walls of the well bore and prevents fluid loss into subterranean zones penetrated by the well bore. However, relatively large openings such as channels, fractures, vugs and the like are often encountered which communicate the well bore with one or more porous subterranean zones. Because of the size of such openings filter cake from the drilling fluid does not seal the openings and drilling fluid is lost from the well bore into the subterranean zones. This in turn causes drilling fluid circulation to be lost whereby the drilling operations must be terminated while remedial steps are taken.
Heretofore, a variety of methods and sealing compositions have been developed and used for combating drilling fluid lost circulation problems. However, such methods and compositions have often been unsuccessful due to inadequate viscosity development by the sealing compositions used. Also, the methods of placement of the sealing compositions have been inadequate to cause plugging of the weak permeable portions of the zones being treated and to prevent bypassing of such portions by the sealing compositions and/or the wash-out of the compositions.
After a well bore penetrating a subterranean hydrocarbon producing formation has been drilled, the well bore is often completed by sealing a string of pipe such as casing or a liner in the well bore. That is, a sealing composition such as a hydraulic cement slurry is pumped into the annular space between the walls of the well bore and the exterior of the string of pipe disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. Sometimes, however, very small openings such as holes or cracks in the well casing and/or the cement sheath are formed through which fluids can undesirably flow into or out of the well bore. Also, very small openings such as channels or fractures can be encountered in a well bore both during drilling and in the completion of the well bore through which drilling and completion fluids are lost. Because of the very small size of such openings in pipe strings, cement and/or the well bore, it is often difficult to seal the openings due to the inability of the sealing composition utilized to enter the openings.
Thus, there is a continuing need for improved methods and sealing compositions for sealing very small openings through which fluids leak into or out of the well bore as well as relatively large openings through which drilling fluid circulation is lost during drilling.